# PHYSICS 231 INTRODUCTORY PHYSICS I

Urban and Civil

Nov 29, 2013 (4 years and 5 months ago)

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PHYSICS 231

INTRODUCTORY PHYSICS I

Lecture 22

Simple Pendulum

Traveling Waves

Longitudinal, Transverse

Sinusoidal wave

Speed, frequency, wavelength

Last Lecture

Speed of a Wave in a Vibrating
String

T

is tension.

Pitch = frequency:

Example 13.9

A string is tied tightly between points
A

and
B

as a communication device. If one wants to
double the wave speed, one could:

a) Double the tension

c) Use a string with half the mass

d) Use a string with double the mass

e) Use a string with quadruple the mass

Superposition Principle

Traveling waves can pass
through each other without
being altered.

Reflection

Fixed End

Reflected wave is inverted

Reflection

Free End

Reflected pulse not inverted

Chapter 14

Sound

Sound Waves

Sound is longitudinal pressure (compression) waves

Range of hearing: 20 Hz to 20,000 Hz

FREQUENCY DEMO

Speed of Sound

Liquids and Gases: B is bulk

modulus,
r

Solids: Y is Young’s modulus

331 m/s is
v

at 0
°

C;

T is the absolute temperature.

Example 14.1

John Brown hits a steel railroad rail with a hammer.

Betsy Brown, standing one mile down the track, hears

the bang through the cool 32

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through the steel by placing her ear on the track.

DATA: Y
steel
=2.0x10
11

Pa,
r
steel
=7850 kg/m
3

What is the time difference between the moments
when Betsy and Boopsie hear the bang?

4.54 s

Intensity of Sound Waves

SI units are W/m
2

Power

Area

Intensity is proportional to square

of amplitude (pressure modulation)

Intensity Range for Human Hearing

Threshold of Hearing

10
-
12

W/m
2

D

-
10

atm !

Threshold of Pain

1.0 W/m
2

Decibel Scale

Sensation is logarithmic

I
0

is threshold of hearing

(0 dB)

Threshold of Pain is

therefore 120 dB

Intensity vs. Intensity Level

INTENSITY

is P/A, W/m
2

INTENSITY LEVEL

is in decibels (dimensionless)

Sound Level Demo

Example 14.2

A noisy machine in a factory produces a sound with a level
of 80 dB. How many machines can the factory house
without exceeding the 100
-
dB limit?

a) 12.5 machines

b) 20 machines

c) 100 machines

Spherical Waves

Energy propagates equally in
all directions

Example 14.3 (skip)

A train sounds its horn as it approaches an intersection.
The horn can just be heard at a level of 50 dB by an
observer 10 km away. Treating the horn as a point source
and neglect any absorption of sound by the air or ground,

a) What is the average power generated by the horn?

b) What intensity level of the horn’s sound is observed by
someone waiting at an intersection 50 m from the train?

a) 126 W

b) 96 dB

Example 14.4

-
W train whistle and figures out
that he won’t have any trouble standing 2 meters
from the whistle since his stereo speakers are rated
at 100 W and he has little trouble with the speakers
turned all the way up. What is the intensity level of
the whistle?

116 dB

Doppler Effect

A change in the frequency experienced by an
observer due to motion of either the observer or
the source.

DOPPLER DEMO

Doppler Effect, Moving Observer

Fig 14.8, p. 435
Slide 12
When not moving,

When moving,

If observer moves away:

Fig 14.9, p. 436
Slide 13
Example 14.5

Mary is riding a roller coaster. Her mother who is
standing on the ground behind her yells out to her at a
frequency of 1000 Hz, but it sounds like 920 Hz.

(v=343 m/s)

What is Mary’s speed?

27.4 m/s

Doppler Effect

Source in Motion

applet

Doppler Effect

Source in Motion

Doppler Effect, Source in Motion

Approaching source:

Source leaving:

Example 14.6

An train has a brass band playing a song on a flatcar. As
the train approaches the station at 21.4 m/s, a person on
the platform hears a trumpet play a note at 3520 Hz.

DATA: v
sound

= 343 m/s

a) What is the true frequency of the trumpet?

b) What is the wavelength of the sound?

c) If the trumpet plays the same note after passing the
platform, what frequency would the person on the
platform hear?

a) 3300 Hz

b) 9.74 cm

c) 3106 Hz

Fig 14.11, p. 439
Slide 15
Shock Waves (Sonic Booms)

When the source velocity approaches the speed of sound,

Both humidity (reflected intensity) and speed of clouds

(doppler effect) are measured.

Doppler Effect:

Both Observer and Source Moving

Switch appropriate signs if observer
or source moves away

Example 14.7

At rest, a car’s horn sounds the note A (440 Hz). The
horn is sounded while the car moves down the
street. A bicyclist moving in the same direction at
10 m/s hears a frequency of 415 Hz.

DATA: v
sound

= 343 m/s.

What is the speed of the car? (Assume the cyclist is
behind the car)

31.3 m/s

Example 14.8a

A train has a whistle with a frequency of a 1000 Hz,
as measured when both the train and observer are
stationary. For a train moving in the positive
x

direction, which observer hears the highest frequency
when the train is at position x=0?

Observer
A

has velocity V
A
>0 and has position X
A
>0.

Observer
B

has velocity V
B
>0 and has position X
B
<0.

Observer
C

has velocity V
C
<0 and has position X
C
>0.

Observer
D

has velocity V
D
<0 and has position X
D
<0.

Example 14.8b

A train has a whistle with a frequency of a 1000 Hz, as
measured when both the train and observer are
stationary. A train is moving in the positive
x

direction. When the train is at position x=0,

An observer with V>0 and position X>0 hears a
frequency:

a)
> 1000 Hz

b)
< 1000 Hz

c)
Can not be determined

Example 14.8c

A train has a whistle with a frequency of a 1000 Hz,
as measured when both the train and observer are
stationary. A train is moving in the positive
x

direction. When the train is at position x=0,

An observer with V>0 and position X<0 hears a
frequency:

a)
> 1000 Hz

b)
< 1000 Hz

c)
Can not be determined

Example 14.8d

A train has a whistle with a frequency of a 1000 Hz,
as measured when both the train and observer are
stationary. A train is moving in the positive
x

direction. When the train is at position x=0,

An observer with V<0 and position X<0 hears a
frequency:

a)
> 1000 Hz

b)
< 1000 Hz

c)
Can not be determined